Content

A request body once it has been encoded for the wire: bytes that know their media type.

The two travel together because a body and its Content-Type written apart can disagree, and the server reads the bytes by the type it is told. The constructor is closed for the same reason: an application gets a Content from an encoder such as Http.form and hands it to post, so the type it carries is always the one its bytes were written in. The bytes are the array that will be sent, so two Contents are not compared by what they hold.